The growth of local and wide area networks based on Ethernet technology has been an important driver for cabling offices and homes with structured cabling systems having multiple twisted wire pairs. The structure cable is also known herein as communication cabling and typically comprises four twisted wire pairs. In certain networks only two twisted wire pairs are used for communication, with the other set of two twisted wire pairs being known as spare pairs. In other networks all four twisted wire pairs are used for communication. The ubiquitous local area network, and the equipment which operates thereon, has led to a situation where there is often a need to attach a network operated device for which power is to be advantageously supplied by the network over the network wiring. Supplying power over the network wiring has many advantages including, but not limited to: reduced cost of installation; centralized power and power back-up; and centralized security and management.
The IEEE 802.3af-2003 standard, whose contents are incorporated herein by reference, is addressed to powering remote devices over an Ethernet based network. The above standard is limited to a powered device (PD) having a maximum power requirement during operation of 12.95 watts. Power can be delivered to the PD either directly from the switch/hub known as an endpoint power sourcing equipment (PSE) or alternatively via a Midspan PSE. In either case power is delivered over a set of two twisted pairs.
The IEEE 802.3at Task Force has been established to promote a standard for delivering power in excess of that described in the aforementioned IEE 802.3af-2003 standard. The IEEE 802.3at Task Force is in the processing of developing a high power standard, to be known as the IEEE 802.3at standard, which enables an increases power delivery over two twisted wire pairs, at least in part by substantially increasing the allowed current. When the increased current is passed through a data transformer, as is typically accomplished in phantom powering, the effective inductance of the data transformer is reduced, primarily as a result of a DC bias current flowing through the data transformer as a result of a current imbalance between each wire in a pair. Such a reduced inductance changes the data signal waveform, particularly by increasing the droop of the signal pulse.
IEEE 802.3-1995 published by the Institute of Electrical and Electronics Engineers, New York, the entire contents of which is incorporated herein by reference, specifies at clause 25.2, through incorporation, ANSI standard X3.263-1995, published by the American National Standards Institute, Washington, D.C., the entire contents of which is incorporated herein by reference. ANSI X3.263-1995 specifies a worst case droop for the driving data transformer of the physical layer level, which defines the minimum inductance of the driving data transformer to be 350 μH for 100 BaseT operation. As described above, as the phantom powering current increases, the imbalance current increases, the transformer DC bias current thus increases, and as a result the effective inductance is reduced, thereby increasing the droop beyond the specified lower limits, resulting in an increased bit error rate (BER).
One solution is to increase the inductance of the data transformer, so that at an increased phantom powering current the effective inductance meets the desired minimum. Unfortunately, increasing the inductance of the data transformer adds cost, and/or requires an increased physical size, and is thus not desirable.
Modern PHY receivers used in switches and PDs are capable of reading data transmitted from a transmitter exhibiting an inductance lower than 350 μH by utilizing digital signal processing techniques. In practice, switches and PD's exhibiting an effective inductance as low as 120 μH, and even lower, may be successfully operated using these techniques. Thus, as the current increases in accordance with the proposed IEEE 802.3at standard, the reduced inductance responsive to the increased DC bias current does not result in an increased BER, as long as the receiver is aware of the expected decreased effective inductance, and is provided with the appropriate digital signal processing techniques. The IEEE 802.3at Task Force has thus required mutual PSE-PD identification/classification which allows a lower inductance only if both the PSE and PD know that they are each of an IEEE 802.3at type, which must be supplied with an appropriate PHY. Older switches and PDs may not exhibit a PHY operative at low driving inductances, and thus they may be unable to reliably read such data, resulting in an increased BER when communicating with a DTE, either a switch or a PD, having such an effective low inductance.
FIG. 1 illustrates a high level block diagram of an arrangement 10 for powering a PD from a switch/hub equipment 30 using phantom powering in accordance with the above proposed IEEE 802.3at standard. Arrangement 10 comprises: switch/hub equipment 30 comprising a first and second data pair 20, a power sourcing equipment (PSE) 40, and a first and second data transformer 50; four twisted pair data connections 60 constituted in a single structured cable 65; and a powered end station 70 comprising a first and second data transformer 55, a first and second data pair 25, and a PD 80.
The primary of each of first and second data transformers 50 carry respective data pairs 20. An output and return of PSE 40 are connected, respectively, to the center tap of the secondary of first and second data transformers 50. The output leads of the secondary of first and second data transformers 50 are respectively connected to first ends of a first and a second twisted pair data connection 60 of structured cable 65. The second ends of first and second twisted pair data connections 60 are respectively connected to the primary of first and second data transformers 55 located within powered end station 70. The center tap of the primary of each of first and second transformers 55 is connected to a respective input of PD 80. Third and fourth twisted pair data connections 60 of structure cable 65 are connected to respective inputs of PD 80 for use in an alternative powering scheme known to those skilled in the art. In another embodiment, third and fourth twisted pair data connections 60 further carry data. First and second data pair 25 are connected (not shown) to PD 80, and represent data transmitted between PD 80 and switch/hub equipment 30.
In operation, PSE 40 supplies power over first and second twisted pair data connection 60, thus supplying both power and data over first and second twisted pair data connections 60 to PD 80. PSE 40 of switch/hub equipment 30 is in direct communication with PD 80 receiving the power. Thus, switch/hub equipment 30, implementing the above described mutual PD-PSE identification/classification, may identify that PD 80 is capable of using high power levels and is equipped with an appropriate PHY capable of handling transmitted signals generated with reduced inductance due to the increased current. PD 80 further identifies that PSE 40 of switch/hub equipment 30 is consonant with high power levels, and thus determines that switch/hub equipment 30 is therefore similarly equipped with an appropriate PHY capable of handling transmitted signals generated with reduced inductance due to the increased current. In the event that PD 80 does not identify PSE 40 as a high power PSE, PD 80 is required to consume low power levels, thereby not reducing the effective inductance of data transformers 50, 55.
FIG. 2 illustrates a high level block diagram of an arrangement 100 for powering a PD from a Midspan PSE in accordance with the above proposed IEEE 802.3at standard over data pairs, sometimes referred to as alternative A powering. Arrangement 100 comprises: a switch/hub equipment 35 comprising a first and second data pair 20 and a first and second data transformer 50; a first and a second set of four twisted pair data connections 60, each constituted in a single structured cable 65; a Midspan power insertion equipment 110 comprising a PSE 40 and a first and a second data transformer 57; and a powered end station 70 comprising a first and second data transformer 55, a first and second data pair 25, and a PD 80.
The primary of each of first and second data transformers 50 carry respective data pairs 20. The output leads of the secondary of first and second transformers 50 are connected, respectively, to the primary of first and second data transformer 57 via a respective first and second twisted data pair connection 60 of first structured cable 65. The center tap of the secondary of first and second data transformer 57 are respectively connected to the output and return of PSE 40. The output leads of the secondary of first and second data transformer 57 are connected, respectively, to the primary of first and second data transformer 55 via a respective first and second twisted data pair connection 60 of second structured cable 65. The center tap of the primary of each of first and second transformers 55 is connected to a respective input of PD 80. Third and fourth twisted pair data connections 60 of first structure cable 65 are connected through Midspan power insertion equipment 110 to third and fourth twisted pair data connection 60 of second structured cable 65, and third and fourth twisted pair data connections 60 of second structured cable 65 are further connected to respective inputs of PD 80 for use in an alternative powering scheme known to those skilled in the art. In another embodiment, third and fourth twisted pair data connections 60 further carry data. First and second data pair 25 are connected (not shown) to PD 80, and represent data transmitted between PD 80 and switch/hub equipment 35.
In operation PSE 40 of Midspan power insertion equipment 110 supplies power to powered end station 70 over first and second twisted pair connections 60 of second structured cable 65, along with data being supplied from switch/hub equipment 35. Power from PSE 40 is of a sufficiently high current to reduce the effective inductance of the windings of data transformers 57 and 55 in which the current of PSE 40 flows. Data transformers 50 exhibit their initial inductance, typically at least 350 μH, due to the fact that there is no load current flowing through them. As a result, data transmitted from switch/hub equipment 35 to PD 80 does not exhibit an increased droop, and the data may thus be read by PD 80 without requiring the above mentioned digital signal processing techniques.
PD 80 performs mutual PSE-PD identification/classification with PSE 40 of Midspan power insertion equipment 110, and thus does not properly identify the capabilities of switch/hub equipment 35. In particular, switch/hub equipment 35 may be incapable of deciphering data transmitted by data pair 25 of powered end station 70 of PD 80, as it exhibits a droop associated with a data transformer of 120 μH due to the effect of the current from PSE 40 in place of the droop associated with a data transformer of 350 μH. Thus, the arrangement of FIG. 2 may result in an increased BER.
What is desired, and not supplied by the prior art, is a mechanism to allow for the use of a Midspan PSE supporting increased power rates above those permitted by IEEE 802.3af-2003, without substantially increasing the BER.